high fructose corn syrup.

obesity becomes the new norm. everyone gets so fat they can't walk around, global production grinds to a halt. Nobody can remember how to hunt or farm since its so long since our sedentary society has engaged in such primitive practices.

the irony is that we all die of starvation in the aftermath of our obesity. slow acting doomsday.

Death by blort.

Stockpile thermite. When sufficient mass is achieved, ignite the lot and burn a hole through the earth's crust. The resulting supervolcanic eruption should end civilization.

Excellent. Classic Dr. Evil. You'll probably need to keep pumping oxygen into the area to make sure it stays on fire.

Thermite is self-oxidizing

Ha, my cousin plans to build a huge fleet of zepplins that play Breathe by Prodigy as he nukes the cities of the world. I'm just hoping he's not serious. (Though that song would be fitting for nuking cities :-) )

He should have some UAVs with speakers flying at lower altitude, otherwise people won't recognize the tune. Also, some heat shielding on the zepplin and a very well-protected cabin -- I'm not sure those things can get out of range fast enough.

Simple. A basic worm that infects IOS-based (Cisco) devices and renders them all inoperable at a coordinated time.

No Intarwebs would either equal doomsday, or massive social liberation.

The death of the internet would hardly result in the collapse of global civilization. We'd just revert to more primal past times, like looting and arson.

...and the library.

Allez card catalog.

An acoustic device that produces high-amplitude waves at the resonant frequency of the human cranial cavity: head goes pop!

Dust Silicon Valley, Cambridge, Manhattan, Pasadena, Austin, and Chapel Hill with prions.